A repetitive electro-optical gate can perform several functions in an optical transmission link. A particularly important function is to generate an output optical pulse stream from a continuous wave optical input. An optical pulse stream is required for transmission in return-to-zero (RZ) format.
Conventional non-return-to-zero (NRZ) format signalling is reaching its practical limits at data rates approaching 10 Gb/s. RZ signalling, superimposing the data on a stream of discrete pulses, has been demonstrated to be more tolerant to impairments due to optical non-linearity, which limit performance in amplified, dispersion managed optical links. The RZ format is therefore likely to be deployed in fixture optical networks, such as dense wavelength-division multiplexed (WDM) optical networks transmitting 10 Gb/s over long spans and planned networks using data rates of 40 Gb/s and above per channel over shorter spans.
An electro-optic gate may also be used to demultiplex a stream of optical pulses. When the electrical drive signal to such a gate is synchronised with the pulse rate B of the stream, the gate may be used to select every nth pulse from the stream, where n is an integer. Such optical demultiplexing allows transmission at data rates exceeding the capabilities of existing electronic components, since the electronic components are only required to operate on a demultiplexed signal at a rate of B/n.
RZ signal generation requires a source of short optical pulses with repetition rate corresponding to the bit rate of the channel along which the signal is to be transmitted. The train of pulses from the source may then be further-gated with an intensity modulator to provide a stream of RZ format data. An important parameter of the source is the duty cycle of the pulse train that it generates. The duty cycle is defined as the ratio of the full-width half-maximum intensity of the pulse, Δ, to the pulse repetition period T. A sharp, distinct pulse thus has a low duty cycle.
The optimum duty cycle of a pulse train depends on the details of the particular transmission system in use. A low duty cycle can minimise impairment due to self phase modulation. Impairments due to cross-phase modulation and adjacent channel crosstalk may be minimised with a duty cycle of around 0.5; but a significantly lower duty cycle can lead to problems with frequency spread. There is thus a need to provide an adjustable RZ source, in which the pulse duty cycle may be adjusted to an optimum level for the properties of the particular system in which it is used.
When several independent RZ data streams are to be multiplexed together to give a higher rate aggregate pulse stream, the duty cycle of each independent RZ data stream must be sufficiently low to prevent overlap of pulses in the aggregate stream after multiplexing. This maintains data integrity and facilitates demultiplexing when the signal is received. To successfully multiplex n data streams, the duty cycle of each stream should be less than ½n. Similarly, to demultiplex a stream of n components, the duty cycle of the optical gate should again be ½n or lower.
Such electro-optical gates are conventionally based on Mach-Zehnder modulators. An electrical signal alters the refractive index along the path taken by one half of a split coherent optical input. The phase of this half is thus altered, such that when both halves are recombined, constructive or destructive interference may occur. A suitable electrical signal input to a Mach-Zehnder modulator thereby operates it as an on/off optical gate. Mach-Zehhnder modulators can be realised in LiNbO3 (lithium niobate), GaAs (gallium arsenide), AlGaAs (aluminium gallium arsenide or other electro-optic materials.
To achieve the lowest duty cycle from a Mach-Zehnder modulator, the modulator is biased for maximum optical transmission and is driven with an electrical signal of amplitude twice the switching voltage. When the input electrical signal is a pure simusoidal oscillator signal, this provides an output of optical pulses at twice the frequency of the input electrical signal, with a duty cycle of about 0.35. This duty cycle is not low enough for many systems, particularly for effective multiplexing, and it is impractical to vary it without harming other essential parameters, such as extinction ratio.
Consideration has been given to using electro-absorption modulators instead of electro-optic gating devices. These have a non-linear light-voltage response and can be induced to achieve low duty cycle gating. However, such devices are extremely limited in the range of optical wavelengths at which they work. Mach-Zehnder modulators can be produced to cover a very broad wavelength range, including those at which current and envisaged optical communications networks are planned to operate.
It is known that a plurality of Mach-Zehnder modulators may be concatenated to produce a pulse stream of lower duty cycle, albeit at the expense of increased complexity and cost. However, such a system is able to operate only at a single non-adjustable duty cycle.
It is therefore an object of the present invention to provide an electro-optical gating device which is operable at a duty cycle lower than that achievable with current electro-optical gating devices and which is capable of adjustment to operate at a duty cycle most appropriate to a given optical data transmission system. It is a further object of the present invention to provide a method for operating such a device.